This invention generally relates to imaging apparatus using spatial light modulators and more particularly relates to an apparatus and method for providing an expanded range of variable light intensity values to a light beam that is modulated by a spatial light modulator.
Two-dimensional spatial light modulators are being widely used in a range of imaging applications from projection of color images to printing of monochrome and color images onto photosensitive media. Because it forms a complete, two-dimensional image at one time without requiring mechanical movement, the spatial light modulator offers a number of advantages over other types of imaging devices, such as scanning lasers, for example.
A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light by transmitting (or reflecting) or by blocking transmission (or reflection) of incident light from a light source. There are two salient types of spatial light modulators that are employed for forming images in projection and printing apparatus. The liquid crystal device (LCD) modulates an incident beam by selectively altering the polarization of light for each pixel. A transmissive LCD operates by selectively transmitting the incident beam through individual array elements. A reflective LCD selectively changes the polarization of a reflected beam at individual array elements. The second basic type of spatial light modulator currently in use is the digital micromirror device (DMD), disclosed in U.S. Pat. No. 5,061,049. The DMD modulates light by reflection at each individual pixel site.
Spatial light modulators were initially developed for digital projection applications. Examples include display apparatus such as those disclosed in U.S. Pat. No. 5,325,137 to Konno et al. and in U.S. Pat. No. 5,743,610 to Yajima et al.; and miniaturized image display, mounted within a helmet or supported by eyewear, disclosed in U.S. Pat. No. 5,808,800 to Handschy et al. Advantageously, spatial light modulators operate by displaying a complete image frame at a time.
More recently, spatial light modulators have been used in printing apparatus, from line printing systems such as the printer disclosed in U.S. Pat. No. 5,521,748 (Sarraf) to area printing systems, such as the printer disclosed in U.S. Pat. No. 5,652,661 (Gallipeau et al.)
It is instructive to consider some of the more important differences between projection and printing requirements for spatial light modulator devices. Effective image projection requires that the image forming device provide high levels of brightness. In display presentation, the human eye is relatively insensitive to many types of image artifacts and aberrations, since the displayed image is continually refreshed and is viewed from a distance. Motion and change also help to minimize the effects of many types of image artifacts. High resolution is not a concern for projection applications, with 72 pixels per inch normally satisfactory for many types of images.
Image printing, meanwhile, presents a number of different problems. For example, when viewing output from a high-resolution printing system, the human eye is not nearly as xe2x80x9cforgivingxe2x80x9d to artifacts, aberrations, and non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. High resolution may require print output at 1200 dpi or higher, depending on the application. For the purpose of the present application, the general term xe2x80x9cimaging apparatusxe2x80x9d is intended to encompass both projection and printing apparatus.
One known limitation with spatial light modulators is that a device has only a limited bit range for addressing, thus can only provide a discrete number of intensity values. Typically 256 intensity values can be addressed and used with conventional spatial light modulators. While this can be sufficient for many imaging applications, there are environments for which the capability to obtain more than this discrete number of intensity values would be an advantage. Applications for which an increased capability for representing various intensity states would include medical imaging, entertainment, and simulation environments.
The spatial light modulator is capable of achieving a range of intensity values, with the actual discrete density value available for a given code value somewhat variable, based on bias voltage provided for the spatial light modulator. A slight change in bias voltage can mean that different intensity values result for the same data.
Conventionally, the bias voltage value used for a spatial light modulator is set at calibration, thereby fixing the set of intensity values in a 1:1 relationship with its corresponding set of input code values. Once calibrated, the spatial light modulator is configured to deliver this set of discrete intensity values, with no change unless recalibrated at a later date.
With earlier spatial light modulators, sluggish device response times precluded xe2x80x9cre-tuningxe2x80x9d or changing bias voltage during imaging operation. Even now, with continuing device development that has resulted in decreased response and settling times, imaging apparatus designers have not taken advantage of the ability to make dynamic changes to spatial light modulator tuning during operation.
Imaging apparatus designs have been proposed with arrangements that use multiple spatial light modulators for printing or projection, even using more than one spatial light modulator per color channel. However, designers have not exploited the capability for obtaining additional output intensities for the same set of pixels within a color channel. Thus, it can be seen that there would be advantages to a spatial light modulator-based imaging system that provides an increased number of light intensity values within a range.
It is an object of the present invention to provide an imaging apparatus using a spatial light modulator for forming an image from input image data wherein the spatial light modulator, at a predetermined bias voltage setting, is capable of providing, at each output pixel in an image, for any one of n input code values any one of n corresponding output intensity levels.
Briefly, according to one aspect of the present invention a method of forming an image having an increased number of output intensity levels m for a group of input data values, where m greater than n, comprises the following sequence:
(a) applying a first bias voltage to the spatial light modulator;
(b) mapping each input image data value in said group of said input image data values to a corresponding first input code value obtained from a first look-up table, wherein said first input code value is selected from a first set containing up to n input code values, and providing each said first input code value to the spatial light modulator;
(c) modulating an incident light beam at the spatial light modulator according to each said first input code value in order to form a first array of output image pixels, wherein the intensity of each output image pixel in said first array of output image pixels is conditioned by each said first input code value;
(d) applying a second bias voltage to the spatial light modulator;
(e) mapping each input image data value in said group of said input image data values to a corresponding second input code value obtained from a second look-up table, wherein said second input code value is selected from a second set containing up to n input code values, wherein said second set contains at least one input code value that is not in said first set, and providing each said second input code value to the spatial light modulator;
(f) modulating an incident light beam at the spatial light modulator according to each said second input code value in order to form a second array of output image pixels, wherein the intensity of each output image pixel in said second array of output image pixels is conditioned by each said second input code value.
In an alternative embodiment, the present invention provides a method for obtaining an increased number of output intensity levels, using only a single look-up table that has more than n possible output intensity levels, by varying the bias voltage to the spatial light modulator among two or more levels. In yet another alternative embodiment, the present invention can be used simply by switching between multiple look-up tables without changing the bias voltage level (that is, omitting step (d) above).
It is an feature of the present invention that it utilizes the response characteristics of a spatial light modulator based on its applied bias voltage, so that a higher bit density can be obtained using a single spatial light modulator device or using multiple devices.
It is an advantage of the present invention that it provides a method that increases the number of available light intensity levels provided by a spatial light modulator, with minimal added cost.
It is a further advantage of the present invention that it can be used with an imaging system that employs a single spatial light modulator or with a system that employs a plurality of spatial light modulators, including systems that use multiple spatial light modulators per color channel.
It is a further advantage of the present invention that it allows the use of different performance settings in an imaging device that uses a spatial light modulator, allowing the imaging device to adapt its behavior to different output media or viewing conditions.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.